If it's an honest question...
How much of the Old Testament applies to Christians is somewhat up for discussion, but the Bilbical answer is "Not all of it, unless you're an Israelite Jew". The Old Testament Law was broken into several sections, but not particularly neatly. There were National laws, Ceremonial laws, Purity laws, and Moral laws.
Purity laws were rather well addressed in Acts and Romans (at least; I'd have to dig through the other Pauline and deuteroPauline letters to find other explanations), where Paul makes a distinction between Jewish obligations (as God's chosen people), and non-Jewish Christians. It's actually a rather extensive, and at the time divisive, argument within the church. He and Peter had a rather intense argument about it in Acts, but the end result is Paul's argument that non-Jewish Christians are, well, not Jewish, and so should not be Jewish... they should be Christian. Even in the Old Testament, non-Jews living in Israel were excluded from obligations and expectations of purity and ceremonial laws, but not from moral or (most) national laws (some national laws doubled with ceremonial, particularly regarding things like Passover). These laws involved what was clean/unclean, processes for sacrifices, Sabbath observation, and showing proper respect for God.
National laws are most of those dealing with punishment, judiciary practices, etc... Those are Israel-specific, and don't translate elsewhere. These tended to involve retribution, debt, forgiveness of wrongs, property, and things of this sort. Sabbath observation fell here, too.
Moral laws are to be obeyed by Christians, and tend to revolve around not worshiping other gods, sexual purity, charitable obligations, and the general "be excellent to one another". Some questions of proper respect for God also fall here, though these are rather hotly debated in the Church whether most of these are universal, or were culture/church specific-- ie, were the commands given because that was how the culture showed respect, or because that is how all Christians are expected to show respect to God.
All three sets of laws were unchanged by Christ's coming and death (save sacrifices-- those were fulfilled forever).
The biggest issue is where Moral laws and Purity laws have their division. Most Christian scholars draw their distinction by what Jesus, Paul, James, Peter, Luke, or John specifically note in the New Testament as verboden as being the Moral Laws, while most of the eating, clothing, crops, etc... things fall under Purity laws-- things that were designed to separate the Jews from other peoples. ("Holy" doesn't actually mean good-- it means set aside for a specific purpose)
To that end, Exodus, Leviticus, Numbers, and Deuterotomy aren't changed, but parts have an Israel-specific audience, parts have a Jewish-people-specific audience, and parts are universal. Which... is pretty much how they always were. Non-Jews in Israel weren't expected to follow purity laws, but they were held to National laws, and they were expected to follow Moral laws
It's not a question of "pick and choose", so much as attempting to identify what Scripture says about each and maintaining Scriptural consistency.
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